Lungs are the principal organs of respiration.Ty lie in thoracic cavity. Right lung is 3-lobed and left lung two-lobed. The cone shaped space between two lungs is called mediastinum. Each lung is covered by a two-layered membrane. Its outer membrane is called parietal pleura and inner visceral pleura. The space between these two membranes is called pleural cavity which is filed with a serous fluid. This fluid lubricates the lungs and protects them from friction.